The present invention is particularly applicable for shaping the outer surface of a cam on a camshaft used in an internal combustion engine and it will be described with particular reference thereto; however, the invention is much broader and can be used for shaping, in a planar direction, many elongated, generally flat surfaces formed in a workpiece of hardenable steel and having a geometrically continuous configuration on the workpiece. For instance, a cam of a camshaft has a continuous elongated surface extending around the camshaft even though this surface has a different radial spacing at the heel of the cam than it does at the nose of the cam. This cam surface is a continuous, elongated surface extending circumferentially around the camshaft. Other similar elongated surfaces can be envisioned for use of the present invention, such as internal generally flat cam surfaces and generally flat surfaces extending in a circular path, such as found on cam face plates. The term "generally flat" used to define a surface means that a flat plane extending across the elongated surface and orthogonal to the elongated direction or path of the surface intersects the surface at a line that is generally straight. This generally straight line of intersection may have a slight bow or curve but still be generally flat, such as a transverse crown on a cam surface.
Turning now to the preferred use of the present invention, the cam surfaces of camshafts must have a given profile laterally across the surface and must have a sufficient hardness to withstand continuous wear from lifter rods or valves. This is especially true with the more strenuous demands of high speed engines using roller lifters. It has been known for many years that the cam surfaces can be inductively heated and then quench hardened to produce the desired surface hardness. In the induction heating process, the temperature of the metal adjacent the cam surface is raised to a value above the critical temperature, A3, of the steel forming the surface so that subsequent quench hardening before cooling causes the desired hardness over the surface. Recently, manufacturers have followed assignees' initiative and have developed machines that position the cam of a camshaft in a circular inductor which inductor, or coil, is then energized by a high frequency (10-25 KHz) at a high power density (20-70 KW/in.sup.2 at the surface) for a short time (less than about 3.0 seconds and preferably less than 2.0 seconds). In this manner, the temperature of the cam surface is increased rapidly for immediate quench hardening by a liquid, such as a low polymer solution. Thereafter, the next adjacent cam is indexed into the heating coil for hardening of that next adjacent surface. The current used is less than about 25 KHz and the heating time is generally less than about 3.0 seconds. The power density in the more recently developed induction heating machinery is about 50 KW/in.sup.2 at the cam surface during the heating operation or cycle. This equipment employs a relatively high power density which causes a rapid increase in temperature adjacent the surface to be hardened. In some instances the camshaft is rotated within the circular inductor during the heating and/or quenching cycle.
In accordance with one relatively successful prior process, the camshaft is indexed to a selected position within the coil so that the nose of the cam faces the insulation gap of the coil, while the heel of the cam is diametrically opposite to this insulation gap. In this indexed position, the heating of the cam surface is controlled. The decrease in heating caused by the gap necessary in most induction heating inductors is offset by the fact that the nose is closer to the surface of the inductor than is the heel. This high energy induction heating process has been highly successful and is utilized often by the automobile industry for hardening the axially spaced cam surfaces of a camshaft for internal combustion engines.
By using this high energy process, higher power density requires a shorter cycle time; therefore, there is a substantial incentive to use the high power density process in the automobile industry. When this process is used to harden cam surfaces, a distinct, transverse crown is created in the surface of a cam after it has been quench hardened. This inherent crown can be in the neighborhood of 0.0010 to 0.0013 in a cam surface which is about 0.50 inches in axial width and heated with an induction heating coil having a diameter of approximately 2.0 inches. Consequently, high power densities for short cycle times have been found to result in a crowning effect for the generally flat cam surfaces. As previously mentioned, "generally flat" indicates a desire for a flat surface, which surface can have a slight amount of crown without affecting the definition. To remove this crown, it has been practice to grind the surface after hardening. This grinding process is followed by a microstoning process for imparting a preselected polished surface to the cam surface. The use of a grinding operation prior to the stoning operation increases the cost and is required when a large crown has been created by the induction hardening process. This inherent crown tends to increase as the power density, which is needed to reduce cycle length, is increased. Desired parameters are, thus, counteracting each other. The hardening process increases the power to reduce cycle length. This higher power increases the inherent crown and, thus, the machining cost and time needed after the hardening process has been completed.
Not only has the high power density, low cycle time induction heating process caused a greater propensity for crowning the surface, but it has also resulted in a hardness pattern which is more shallow at the lateral edges of the cam surface. Since the requirements by customers regarding hardness patterns generally demand a maximum depth to the hardness pattern over the total surface, the uneven hardness pattern resulting from the high density, low cycle time process has required a reduced induction heating frequency to guarantee a minimum hardness depth over the entire surface. This has exaggerated the crowning effect previously discussed.